Aluminum-silicon alloys have been made for a long time by simply adding crushed silicon metal or a high-silicon aluminum base master alloy to a body of molten aluminum. Thus, in one such approach, an electrolytic reduction cell is operated normally to produce aluminum and then solid silicon is added to the aluminum in the cell, or to the aluminum in a holding furnace after recovering it from the cell. Performing the alloying operation in a holding furnace has certain advantages because the melting of fairly large pieces of silicon at the temperature of molten aluminum in a reduction cell takes quite a while, and the heat required tends to upset the cell or interfere with its normal operating behavior.
It has also been proposed to make aluminum-silicon alloys by adding sand or another suitable source of silica to molten aluminum for reaction therewith to reduce the silica to silicon metal. Practicing this in aluminum reduction cells has been difficult for various reasons, although some success has been achieved by carefully controlling the silica additions and reducing the normal feeding of alumina into the molten salt bath of such cells to compensate for alumina generated by the reaction of aluminum with the added silica.
One of the remaining problems with which the present invention is concerned, however, is that the direct addition of sand to an aluminum reduction cell often results in the formation of such heavy ridges of deposited solids along the bottom of the cell, especially between the sidewalls of the cell and the anode, that the cell soon becomes inoperable, sometimes within a few days and often within a week or two.
These ridges are to be distinguished from those of an essentially cryolite-alumina composition encountered in ordinary reduction cell operations. The latter ridges are more readily avoided or more easily redissolved, usually by changing the anode-cathode spacing to increase the cell voltage and raise its bath temperature. The ridges presently referred to involve an accumulation of solid material separating from the liquid cryolite-alumina-silica system, that seems to build up from and upon the conventional formations. There have been occasions when this deposited material has been so excessive as to restrict downward movement of the anode, and operation of the cell could be continued only by running at increased voltage in an inefficient manner. These deposits have also caused anode-cathode short circuits which reduce production.